Sex steroid hormones, such as testosterone (T) and 17beta-estradiol (E2) are well known to regulate both behavior and physiology. Many of these effects require that the steroids act on the brain by finding to their steroid receptor proteins that are distributed throughout the brain. An important unanswered question facing the field of neuroendocrinology concerns how steroid action is coordinated at multiple levels. The vocal control system of songbirds is an excellent model system for such components of this circuit that are important in the motor output of song. These include telencephalic nucleic, brainstem nuclei, and the vocal production organ the syrinx. The telencephalic nuclei exhibit a hierarchal organization of motor controls. It is not known if gonadal hormones must act in all these individual nuclei and/or muscle groups simultaneously to produce the full complement of song and whether steroids have independent effects in these different structures. I will investigate steroid action in the song circuit with the selective administration of steroid receptor antagonists centrally and peripherally.